The applications that run on computers typically operate under an operating system (OS) that has the responsibility, among other things, to save and recall information from a hard disk. The information is typically organized in files. The OS maintains a method of mapping between a file and the associated locations on a hard disk at which the file's information is kept. storage. Periodically a backup (copy) is typically made of the disk to address two types of problems: First, the disk itself physically fails making the information it had contained inaccessible. Second, if the information on disk changes and it is determined the original state was desired, a user uses the backup to recover this original state. Backups can be made to the same disk or to an alternate media (disk, tape drive, etc.).
The present invention provides a method and apparatus for information recovery focusing, in one example embodiment, on the second situation not involving a physical disk failure, but where information is altered and access to its original state may be desired. Some typical examples would be: a computer system "crashing" during an update of a piece of information, thus leaving it in neither the original or "new" state, the user changing information only later to desire to restore (or just reference) the original state, a computer virus altering information, or a file being deleted accidentally.
The following are established backup methods and systems:
1. Tape Backup PA1 2. Optical Disk Backup (WORM) PA1 3. RAID Systems PA1 4. Tilios Secure Filing System PA1 5. File Copies
Tape backup traditionally involves duplicating a disk's contents, either organized as files or a disk sector image, onto a magnetic tape. Such a tape is typically removable and therefore can be stored off-site to provide recovery due to a disk drive malfunction or even to an entire site (including the disk drive) being destroyed, for example, in a fire.
When information is copied from a disk to tape in the form of a sector level disk image (i.e., the information is organized on the tape in the same manner as on the disk), a restoration works most efficiently to an identical disk drive. The reason for such an organization is speed. Reading the disk sequentially from start to end is much faster than jumping around on the disk reading each file one at a time. This is because often a file is not stored continuously in one area of the disk, but may be spread out and intermixed with other files across the entire disk. When information is copied one file at a time to a tape it is possible to efficiently restore one or more files to a disk that may be both different and already containing data (i.e., when restoring a saved disk image all prior data on a disk is overwritten).
Tape backup focuses on backing up an entire disk or specific files at a given moment in time. Typically the process will take a long time and is thus done infrequently (e.g., in the evening). Incremental backups involve only saving data that has changed since the last backup, thus reducing the amount of tape and backup time required. However, a full system recovery requires that the initial full system backup and all subsequent incremental backups be read and combined in order to restore to the time of the last incremental backup.
The key shortcoming of tape backup is that you may not have performed a recent backup and therefore may lose the information or work that was subsequently generated. The present invention addresses this problem by employing a new method of saving changing disk information states providing for a continuously running disk backup system. This method could be implemented on a tape drive, as a tape drive does share the basic random read and write abilities of a disk drive. However, it would not be practical for the same reasons a tape drive when used as a disk is generally not very effective: extremely slow random access times.
Write-once optical disk backup as performed by a WORM drive has many of the same qualities as tape backup. However, because of the technology involved, it is not possible to overwrite data. Therefore it provides some measure of a legal "accounting" system for unalterable backups. WORM drives cannot provide continuous backup of changing disk information because eventually they will fill.
A RAID system is a collection of drives which collectively act as a single storage system, which can tolerate the failure of a drive without losing data, and which can operate independently of each other. The two key techniques involved in RAID are striping and mirroring. Striping has data split across drives, resulting in higher data throughput. Mirroring provides redundancy by duplicating all data from one drive on another drive. No data is lost if only one drive fails, since the other has another copy.
RAID systems are concerned with speed and data redundancy as a form of backup against physical drive failures. They do not address reverting back in time to retrieve information that has since changed. Therefore RAID is not relevant to the present invention other than being an option to use in conjunction with the present invention to provide means for recovery from both physical disk drive failures as well as undesired changes.
The Tilios Operating System was developed several years ago by the assignee hereof. It provided for securing a disk's state and then allowing the user to continue on and modify it. The operating system maintained both the secured and current states. Logging of keystrokes was performed so that in the event of a crash, where the current state is lost or becomes invalid, the disk could easily revert to its secured state and the log replayed. This would recover all disk information up to the time of the crash by, for example, simulating a user editing a file. The secured disk image was always available along with the current so that information could be copied forward in time-i.e., information saved at the time of the securing backup could be copied to the current state.
The Tilios Operating System could perform a more rapid backup because all the work was performed on the disk (e.g., there was no transfer to tape) and techniques were used to take advantage of the incremental nature of change (i.e., the current and secured states typically only had minor differences). Nonetheless, the user was still faced with selecting specific times at which to secure (backup) and the replay method for keystrokes was not entirely reliable for recreating states subsequent to the backup. For example, the keystrokes may have been commands copying data from a floppy disk or the Internet, both of whose interactions are beyond the scope of the CPU and disk to recreate.
Simply creating a backup a file by making a copy of a file under a new name, typically changing only a file's extension (e.g., "abc.doc" is copied to "abc.bak") has been a long standing practice. In the event the main file (abc.doc) is corrupted or lost, one can restore from the backup (abc.bak). This process is much the same as doing a selective tape backup and carries the issues of managing the backups (when to make, when to discard, etc.).
In summary, a RAID system only deals with backup in the context of physical drive failures. Tape, WORM, Tilios, and file copies also address backup in the context of recovering changed (lost) information.
No Specific Backup Request or Time
The traditional backup process involves stopping at a specific time and making a duplicate copy of the disk's information. This involves looking at the entire disk and making a copy such that the entire disk can be recreated or specific information recalled. This process typically involves writing to a tape. Alternatively, a user may backup a specific set of files by creating duplicates that represent frozen copies from a specific time. It is assumed the originals will go on to be altered. This process typically involves creating a backup file on the same disk drive with the original. Note that a "disk" may actually be one or more disk drives or devices acting in the manner of a disk drive (storage means).
In both of these cases the user must make a conscious decision to make a backup. In the second case a specific application, like a text editor, may keep the last few versions of a file (information). However, this can lead to wasted disk space as ultimately everything is duplicated long after files have stabilized. In other words, while working on a document a user may likely want to revert to a prior version, but once finished and years later, it is very unlikely the user would care to re-visit the last state before final.
The technology of the present invention seeks to eliminate the need to pause and make backups or decide which files should be backed up in the context of short term information recovery. That is, recovering information that was known reasonably recently as opposed, for example, to recovering information that has been lost for a long period of time.
Backup of a Disk's Directory is Important
Another situation where information recovery is very important is when the directory system for a disk, which identifies what and where files are located on disk, gets corrupted. This occurs, for example, due to a system crash during the directory's update or due to a bug in the operating system or other utility. In either case, losing the directory of a disk's contents results in losing the referenced files, even though they still exist on the disk. In this case the information the user wants to restore is the disk's directory.
A final example of why a user would want to revert to a backup is when the operating system gets corrupted (the executable or data files that are essential to run a computer) due, for example, to installing new software or device drivers that don't work.
Clearly there are many reasons a user might want to go back in time in the context of information being manipulated on a computer's disk. Traditional backups offer recovery to the time of the backup. However, these system-wide backups are limited in frequency due to the amount of time required to scan the disk and duplicate its contents. In other words, it is not feasible to backup an entire disk every few minutes as this would require significant pauses in operation and an enormous amount of storage. Keeping historical copies of files as they progress in time has the drawback of eventually forcing the user to manage the archives and purge copies in order to avoid overflowing the disk. Obviously, one cannot keep a backup of all files on a disk whenever they are changed for all of time without requiring an unlimited disk, which does not exist.
One approach to retaining discarded data on a more or less continuous basis is described in U.S. Pat. No. 5,325,519, entitled "Fault Tolerant Computer with Archival 0 Rollback Capabilities", to Long et al. ("'519 patent"). The '519 patent discloses a storage device which includes processing circuitry for detecting access requests to alter data in respective locations of a storage device, and, prior to executing such requests, storing the data in such locations in an audit partition region of the storage device. The device of the '519 patent can subsequently restore the data retained in the audit partition region to its previous location on the device, and thereby return the storage device to a previous state. The device and approach of the '519 patent, however, inherently introduces delays in writing data to the storage device. In some cases, these delays may make it infeasible to use this technology. Therefore, there remains a need for a more fast, flexible and dynamic way to retain historical information in a computer system.